It's a bit complicated, but basically, it's the net calorie deficit - expending more energy than you consume - that counts, said Eric Ravussin, a physiologist at the Pennington Biomedical Research Center in Baton Rouge, La.
In a small, randomized, controlled clinical trial, Ravussin divided three dozen overweight but healthy men and women into three groups. One group reduced their calorie intake by 25 percent. Another group cut calories by half as much (12.5 percent) while increasing energy output through exercise by 12.5 percent; and the third group made no diet or exercise changes.
The researchers looked at weight loss, body composition, and measures of superficial and deep fat. They found that it doesn't matter whether people lose weight by diet or by exercise or a combination, although exercise has the important benefit of improving cardiovascular health.
"So long as the energy deficit is the same, body weight, fat mass, and abdominal fat will all decrease the same way," said Ravussin, in an e-mail.
It's a good idea to include exercise in any weight control program, said William J. Evans, director of the Nutrition, Metabolism and Exercise Laboratory at the University of Arkansas for Medical Sciences.
"When you exercise during weight loss, you lose more fat from the visceral stores - around the waist because this fat is very metabolically active and turns over more rapidly," he said in an e-mail. There's some evidence that this fat is used first by the body.
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